The present invention generally relates to digitally controlled variable gain circuits, and more particularly to a digitally controlled variable gain circuit which attenuates or amplifies an analog input voltage depending on a digital control value. The digitally controlled variable gain circuit is sometimes also referred to as an electronic volume.
Recently, due to the improved performances of various electronic equipments for public and industrial uses, there are now demands for making fine and highly accurate adjustments of signal levels within a circuit.
It is effective to digitize the electronic equipment in order to improve the performance thereof, however, it is impossible to treat all signals as digital quantities. For example, when an audio signal or the like is converted into a digital quantity, it is impossible to reproduce the original signal with a 100% fidelity, although dependent on the sampling frequency and conversion technique used.
Accordingly, an analog processing part is in many cases included in a part of the digital equipment, and a digitally controlled variable gain circuit is used in this analog processing part to adjust the level of the analog signal.
FIG. 1 shows an example of a general application of the digitally controlled variable gain circuit. In this example, an analog block 1 generates an analog signal 2 or carries out a predetermined process on a signal to form the analog signal 2. This analog signal 2 is input to a digitally controlled variable gain circuit 3. An analog signal 4 output from the digitally controlled variable gain circuit 3 is input to an analog block 5 of the next stage.
A digital control signal 6 having a plurality of bits is applied to the digitally controlled variable gain control circuit 3. The digitally controlled variable gain control circuit 3 generates the analog signal 4 by attenuating or amplifying the analog sign. 2 depending on the content of the digital control signal 6. The attenuation or amplification can finely be controlled in digital quantities, and the level of the analog signal 4 can be adjusted finely and with a high accuracy.
FIG. 2 shows a first example of the conventional digitally controlled variable gain circuit 3. One example of this type of variable gain circuit is proposed in a Japanese Laid-Open Patent Application No. 63-156410.
The digitally controlled variable gain circuit shown in FIG. 2 includes a resistor string 6 made up of n resistors R1, R2, . . . , Rn which are connected is series, n switches S1 through Sn, and a decoder ? . An analog input voltage V.sub.IN is applied across the ends of the resistor string 6, and one of divided voltages from nodes connecting the resistors i output via one of the switches S1 through Sn which turns ON responsive to an n-bit output of the decoder as an analog output voltage V.sub.OUT. In other words, one of the n kinds of voltages including the analog input voltage V.sub.IN is selected and output as the analog output voltage V.sub.OUT depending on a control signal which is input to the decoder ? . The voltage varying step is dependent on the number n of the resistors and switches.
However, according to the first example of the conventional digitally controlled variable gain circuit, there was a problem in that the scale of the circuit becomes large compared to the control resolution because the number of voltage varying steps is dependent on the number n of the resistors and switches.
FIG. 3 shows a second example of the conventional digitally controlled variable gain circuit 3. Examples of this type of variable gain circuit are proposed in Japanese Laid-Open Patent Applications No. 62-173809 and No. 3-255722.
The digitally controlled variable gain circuit shown in FIG. 3 generally includes a first stage part 9 and a second stage part 12 which are coupled via an operational amplifier 10. The first stage part 9 is made up of a first resistor string 8 having n resistors R11, R12, . . . , R1n connected in series, and n switches S11, S12, . . . , S1n. The operational amplifier 10 is operated by high and low power source voltages +V and -V. The second stage part 12 is made up of a second resistor string 11 having n resistors R21, R22, . . . , R2n connected in series, and n switches S21, S22, . . . , S2n.
According to this second example of the conventional digitally controlled variable gain circuit, the analog input voltage V.sub.IN can be attenuated by the first stage part 9, and further attenuated by the second stage part 12. Hence, it is possible to make the voltage varying step correspond to a product of the number of varying steps of the first stage part 9 and the number of varying steps of the second stage part 12. But in this case, an output of the first stage part 9 is applied to a non-inverting input terminal of the operational amplifier 10, and a voltage from the second resistor string 11 is applied to an inverting input terminal of the operational amplifier 10. For this reason, there was a problem in that an operation point of the operational amplifier 10 falls outside the tolerable operation range if the operational amplifier 10 is operated by the lower power source voltage of 2 V, for example.
Generally, the tolerable operation range of the operational amplifier is given between the potential which is approximately 1 V lower than the high potential side power source voltage +V and the potential which is approximately 1 V higher than the low potential side power source voltage -V. For example, if +V=5 V and -V=0 V, the tolerable operation range becomes approximately 3 V. Accordingly, if the operational amplifier is operated by a lower power source voltage of approximately 2 V, it is only possible to obtain an operation range which is extremely narrow and corresponds to an approximate middle potential of the power source voltage.
In addition, if the operational amplifier uses an offset voltage, there is also a problem in that the offset voltages changes when the feedback resistance of the operational amplifier is varied.